Method preparing phosphonic acid derivatives

ABSTRACT

Processes for the preparation of ( + OR - )-(cis-1,2epoxypropyl)phosphonic acid, amides, esters and salts thereof by photolytic rearrangement of 1,2-propenyl phosphates using ultraviolet radiation are disclosed. Also disclosed is the photolytic rearrangement using ultraviolet radiation and the inclusion of a photosensitizing agent. The compounds thus produced are useful as antimicrobial agents.

United States Patent Chemerda et al.

[451 Apr. 25, 1972 [54] METHOD PREPARING PHOSPHONIC ACID DERIVATIVES[72] Inventors: John M. Chemerda, Watchung; William C.

Lumma, Plainfield, both of NJ.

[73] Assignee: Merck & Co., Inc., Rahway, NJ.

[22] Filed: Aug. 31, 1970 [21] Appl. No.: 68,562

3,496,080 2/ l 970 Harris ..204/ l 58 Primary E.taminer-Howard S.Williams Attorney-I. Louis Wolk, J. Jerome Behan and John FrederickGerkens [57] ABSTRACT Processes for the preparation of(:)-(cis-l,2-epoxypropyl)phosphonic acid, amides, esters and saltsthereof by photolytic rearrangement of 1,2-propenyl phosphates usingultraviolet radiation are disclosed. Also disclosed is the photolyticrearrangement using ultraviolet radiation and the inclusion of aphotosensitizing agent. The compounds thus produced are useful asantimicrobial agents.

5 Claims, No Drawings METHOD PREPARING PHOSPI'IONIC ACID DERIVATIVESSUMMARY OF THE INVENTION This invention relates to a novel method forthe preparation of salts, amides and esters of(:)-(cis-1,2-epoxypropyl)phosphonic acid and ()-(cis-l,2-epoxypropyl)phosphonic acid which are useful respectively asantimicrobials and as intermediates in the preparation of (i): (cisl,2-epoxypropyl)phosphonic acid and ()-(cis-l ,2-epoxypropyl)phosphonicacid.

In accordance with this invention (i-)-(cis-l,2-epoxypropyl)phosphonicacid and amides, esters and salts thereof are prepared by photolyticallyrearranging the corresponding 1 ,Z-propenylphosphates. The (i)-( cisl,2-epoxypropyl)phosphonic acid and amides, salts and esters thereof areprepared vby irradiating the corresponding l,2-propenylphosphate withultraviolet radiation. In the reaction scheme below thel,2-propenylphosphate compound (I) is in tautomerie equilibrium with the2-phosphono-l-propanal compound (II). In order to avoid duplication ofnomenclature, throughout this specification the l,2-propenyl phosphateterminology will be employed. However, it is to be understood that theuse of said single name over the other does not preclude the use of thepropanal compounds in the instant process. The irradiation beingincident upon either isomer will result in the formation of a singleproduct (III). The rearrangement is best shown by the following reactionscheme:

wherein if is hy drogen, loweralkoxy, loweralkenyloxy, aryl, for examplephenyl and substituted phenyl in which the substitution is loweralkyland halogen; aralkoxy such as benzyloxy and substituted benzyloxy inwhich the substitution is loweralkyl or halogen;loweralkanoyloxymethoxy; OM where O is oxygen and M is a metal, or aminecation, preferably the cation derived from an alkali metal or analkaline earth metal such as the cation derived from sodium, potassium,lithium, magnesium, or calcium; an amine or substituted amine in whichthe substitution is loweralkyl, and also in which an alkylene linkagemay interconnect an oxygen of X and an oxygen of Y, forming a cyclicdioxy derivative; and an alkyl or aralkyl ammonium salt such as methylammonium, ethyl ammonium, benzyl ammonium and phenethyl ammonium salts.Y is defined in the same manner as is X although in a single compound, Yneed not necessarily be the same as X.

When, in the instant application, reference is made to lower as inloweralkoxy" or loweralkyl, what is meant is that the carbon chainconsists of from 1 to 5 carbon atoms in which said chain may be straightor branched. Included in the definition are methoxy, ethoxy, propoxy,tertiary butoxy, amyloxy, methyl, ethyl, isopropyl, butyl, amyl, and thelike. When reference is made to halogen what is meant is fluorine,chlorine, bromine, and iodine. The substituted aryl and aralkyl groupsabove include substitution at the ortho, meta, and para positions of thebenzene nucleus as well as multiple substitutions thereon. When X and Yare defined as being divalent alkaline earth metals such as calcium,they are construed to be one and the same with both oxygen atoms of thephosphonic acid ionically bonded to a single divalent metallic ion.

The process of the instant invention, when effected by irradiation ofthe starting material (I or II), recovers the racemic mixture ofproducts III which may be represented by the symbols (i) or (d1). Thisis taken by those. skilled in the art to mean that the product is a50:50 mixture of two optically pure compounds, each of which is themirror image of the other, one compound being represented by the symbol(F) and the other by the symbol depending on their optical rotation.

PREFERRED EMBODIMENTS OF THE INVENTION In one embodiment of theinvention, l,2-propenyl phosphate is rearranged by dissolving it in asuitable, nonreactive solvent and, treating it with high intensitylight. The solvents found to be suitable are lower molecular weightorganic solvents. Those preferred solvents have been found to beloweralkanols such as methanol, ethanol, propanol, and the like; lowerketones such as acetone or methyl ethyl ketone; ethers and diethers suchas diethyl ether and l,2-dimethoxy ethane; hydrocarbon solvents, of upto 8 carbon atoms both linear and cyclic such as pentane, hexane orcyclohexane; and aromatic solvents of up to 8 carbon atoms such asbenzene or toluene. Both the intensity and wavelength of the irradiationwill determine the duration of the reaction, and the temperature atwhich it is run. It has been found best to utilize a mercury arc lamp ofgreater than or equal to 200 watts output at a temperature of from roomtemperature to the reflux temperature of the particular solventemployed, for a duration of from 1 to 24 hours. The reaction isgenerally complete in from 5 to 15 hours in a solvent refluxing at atemperature of from 50 to 100 C. Isolation of the product is effected bystandard laboratory techniques known to those skilled in the art such asevaporation of solvents and recrystallization of residues, to affordpure products.

The particular radiation preferred for the rearrangement is that fromamercury vapor lamp of either the high or low pressure type. However,other light sources including sunlight have been found useful. I

The wavelength of light emanating from the irradiation source may becontrolled by the choice of light source itself such as the continuousbroad spectrum of light present in sun.- light or the more narrowselection of wavelengths from mercu ry vapor lamps. The frequency ofirradiation may be made even more narrow or selective by the use offiltering elements which absorb all but certain wavelengths which aretransmitted. These filtering elements may be composed of partiallyabsorbing glass or other substance, chemical solutions, or a combinationthereof. The wavelength of light which is to be usedin general dependson the substituents X and Y. The nature ofX and Y will affect. thechromophore of I and II and thus affectthe wavelength of light theparticular molecule will absorb. In general, only wavelengths longerthan I nanometers and shorter than 400 nanometers will be useful, ananometer being equivalent. to one-billionth of a meter of l millimicronandabbreviated herein as nm.

In a variation of the above process the irradiation proceeds with theinclusion of a photosensitizing agent. This photosensitizing agent inmany cases may be the solvent employed in the reaction or it may beaseparate agent added to the solvent which serves as a photosensitizingagent. The purpose of the photosensitizing agent is to absorb the energybeing in- I troduced into the system by the irradiation and to transferthat energy to the l,2-propenylphosphate (I) or 2-phosphono-lpropanal(II) thereby effecting the rearrangement to the(cisl,2-epoxypropyl)phosphonic acid and derivatives thereof. Thisenergy, after being absorbed by the photosensitizing agent will usually,but not always, be transferred tothe starting materials I and II bymeans of molecular collisions. This process is known to those skilled inthe art as photochemical energy transfer or sensitization.

Photochemical sensitization is capable of producing reactive excitedstates of the l,2-propenylphosphates. (I) or phosphonopropanals (ll)which may not be formed by direct irradiation. The photosensitizingagent A may absorb a wavelength of light the starting material isincapable of absorbing, however, the excited photosensitizing agent maybe very capable of transferringthis absorbed energy to the startingmaterial. Thus the photosensitizing agent broadens the choice ofwavelengths available with which to affect the rearrangement. Although acertain compound only may absorb very few wavelengths, with the properchoice of photosensitizing agent a much wider choice of wavelengths isavailable.

Upon irradiation, the photosensitizing agent may be pushed into manytypes of intermediate reactive states. Among them can be a singletstate, a triplet state, or charge transfer state; they may formexcimers, photoionized or dissociated species, and the like. Theaddition of inorganic salts and other reagents has been found useful inthat the stability of the intermediate excited state is affected andsuch a longer or shorter duration of the excited state may have aneffect upon the outcome of the reaction.

The photosensitizing agent is generally selected from organic compoundssuch as loweralkanones such as acetone, methylethyl ketone, (or3-pentanone); aromatic ketones such as acetophenone, benzophenone; orphenyl cyclopropyl ketone, aromatic hydrocarbons such as benzene,naphthalene, and the like.

Therearrangement reactions of the instant invention are seen to workequally well when the starting material is the acid or the salt or esterderivative of the l,2-propenylphosphate. The substitution on thephosphate group of the starting material is not altered during thereaction but the substitutions on the phosphonate group of the productmay, if desired, be changed to other groups.

The instant process is intended to include other functionally equivalentmethods of preparation. Therefore, any modification of this synthesiswhich results in the formation of an identical product should beconstrued as constituting an analogous method. The claimed process iscapable of wide variation and modification and, therefore, any minordeparture therefrom or extension thereof is considered as being withinthe scope of this invention.

The following examples are included in order that the invention shall bemore fully understood. They are not included for purposes of limitationof the invention.

EXAMPLE 1 Calcium-(:Hcisl ,2-epoxypropyl)phosphonate A. A slurry of 1.82g. (0.01 Moles) of disodium-cis-propenylphosphate in 500 ml. of water ina quartz flask fitted with a nitrogen gas sparger is irradiated under Nat a distance of 3 cm. with a 500 watt high pressure quartz mercury arcfor 26 hours at 50C. The reaction mixture is then cooled and the solventevaporated in vacuo. The residue is dissolved in methanol, treated withdecolorizing carbon and the solvent then re-evaporated. On cooling,spontaneous crystallization yieldsdisodium-(:t)-(cis-l,2-epoxypropyl)phosphonate. A small amount of the(:)-trans isomer is formed presumably due to photochemical isomerizationof the starting cis to trans propenylphosphate.

B. A stirred solution solution of 1.82 g. (0.01 Moles) ofdisodium-cis-propenyl phosphate in 400 ml. of a 1:1 mixture of methanoland acetone is irradiated in a pyrex flask under nitrogenfor two dayswith a 450 watt high pressure immersion lamp surrounded by a 2 mm thicksleeve of vycor glass to filter out wave lengths less than 240 nm.Evaporation of the solvent in vacuo followed by dilution with 100 ml. ofmethanol and decolorization with carbon results in a solution which,upon the addition of 1.58 g. (0.01 Moles) of calcium acetateprecipitates calcium-(:H cisl ,2-epoxypropyl)phosphonate. Filtration ofthe product followed by recrystallization from ethanol affords purematerial. In this experiment acetone absorbs most of the incident lightand functions as a sensitizer, transferring its energy of excitation tostarting propenylphosphate forming an excited state of the latter whichthen reacts.

EXAMPLE 2 Calcium-(1H cisl ,2-epoxypropyl)phosphonate A. Diethyl-(:)-(cisl ,2-epoxypropyl )phosphonate A solution of 19.4 g. 0.1 Mole) ofdiethyl-cis-propenylphosphate in 100 ml. of methylbenzoate is irradiatedunder nitrogen in a quartz vessel with the mercury lamp of Example 18for 24 hours maintaining the temperature at 85 to l00C.

with an external steam bath. The solvent is removed by fractional vacuumdistillation. This is followed by a fraction of purediethyl-(i)-(cis-l,Z-epoxypropyl)phosphonate, boiling point 75 to78C./0.6 mm. Hg. 7

When in the above procedure dimethyl-cis-propenylphosphate ordi-propyl-cis-propenylphosphate is employed in place ofdiethyl-cis-propenylphosphate, there is obtained dimethyl-(:)-( cisl,2-epoxypropyl)-phosphonate and dipropyl-(:Hcisl,2-epoxypropyl)-phosphonate, respectivey B. (::)-(cisl,2-epoxypropyl)phosphonic acid I Thediethyl(i)-(cis-l,2-epoxypropyl)phosphonate is dissolved in 10 ml. oftrimethylchlorosilane and refluxed for eight hours. On cooling, thereaction mixture is diluted by the addition of ice water to yield anaqueous solution of (i)-(cis-l,2- epoxypropyl)phosphonic acid. Thesolution can be evaporated to afford the free acid which melts at 80C.or the solution may be used as is in the next step.

When in the above proceduredimethyl-(i)-(cis-l,2-epoxypropyl)phosphonate and dipropyl -(:)-(cis-l,2-epoxypropyl)phosphonate are employed in place ofdiethyl-(cisl,2-epoxypropyl)phosphonate there is obtained (:)-(cis-l,2-epoxypropyl)phosphonic acid in comparable yields.

C. Calcium-(:Hcis-l ,2-epoxypropyl)phosphonate The aqueous solution of Babove is adjusted to a pH of 8.0 with 10% sodium hydroxide. The mixtureis diluted with methanol and 1.58 g. (0.01 Mole) of calcium acetate andadded to the solution. The precipitate is filtered and recrystallizedfrom ethanol to afford purecalcium-(:Hcis-l,2-epoxypropyl)phosphonate.

EXAMPLE 3 (+)-a-Phenethylammonium salt of ()-(cisl ,2-epoxypropyl)-phosphonate A. Dibenzyl-(:)-(cisl ,2-epoxypropyl)phosphonate 'A stirredsolution of 6.4 g. (0.02 Mole) of dibenzyl-cispropenylphosphate in 100ml. of cyclohexane is irradiated witha 500 watt immersion high pressuremercury lamp at a temperature of 80C. (reflux temperature of thesolvent). The

. solvent is evaporated and the residue, on crystallization frombenzene, affords dibenzyl-(:)-( cis-l ,Z-epoxypropyl)phosphonate.

When in the above procedure di-(p-chlorobenzyl)-cispropenylphosphate anddi-(o-tolyl)-cis-propenylphosphate are employed in place ofdibenzyl-cis-propenylphosphate, there is obtaineddi-(p-chlorobenzyl)-(i)-(cis-epoxypropyl)- phosphonate anddi-(o-tolyl)-(- '-)-(cis-l,2-epoxypropyl)- phosphonate, respectively. p

B. (+)-a-phenethylammonium salt of ()-(cis-l,2-epoxypropyl)phosphonateThe material from A above is dissolved in 40 ml. of ethanol pure and tothis is added 0.5 g. of 5% palladium on charcoal. The

mixture is shaken with hydrogen under 45 psi. at room temperature untilno further hydrogen uptake is observed. The reaction is filtered and tothe filtrate is added 2.42 g. (0.02 Mole) of (+)-a-phenethylamine. Thesolution, on concentration to a small volume gives the(+)-a-phenethylammonium salt of ()-(cis-l,2-epoxypropyl)-phosphonate,melting point 169 to 171C.

When in the above procedure Raney nickel or Adams catalyst is used inplace of 5% palladium on charcoal there is obtained, in comparableyields, the same product.

When in the above proceduredi-(p-chlorobenzyl)-(i)-(cisl,2epoxypropyl)phosphonate anddi-(o-tolyl)-(:)-(cis-l,2- epoxypropyl)phosphonate are employed in placeof dibenzyl- (1-)-(cis-l,2-epoxypropyl)phosphonate there is obtained the(+)-a-phenethylammonium salt of (-)-(cis-l ,2-epoxypropyl)phosphonate incomparable yield.

EXAMPLE 4 Benzylammonium-(i-)-(cis-l ,2-epoxypropyl)phosphonate A.di-Tertiarybutyl-( cis- 1 ,2-epoxypropyl)phosphonate A slurry of 5.6 g.(0.02 Mole) of di-tertiary-butyl-(cisprpenyl)phosphate in 100 ml. oftetrahydrofuran is heated to reflux and the resulting solutionirradiated for 6 hours with a 500 watt mercury immersion lamp under NThe reaction mixture is treated with decolorizing carbon and filtered toafford a nearly colorless solution of di-tertiary butyl phosphonateester. The solution can be evaporated to dryness to afforddi-tertiarybutyl-(cis-l,2-epoxypropyl)phosphonate, or used as is in thenext step.

B. Benzylammonium-(:)-(cis-1,2-epoxypropyl)phosphonate The solution of Aabove is treated with 100 mg. of methane sulfonic acid for 15 minutes atreflux temperature. The solution is cooled and neutralized with 2.14 g.(0.02 M.) of benzylamine. The solution is slowly evaporated untilcrystallization is observed. The suspension on cooling affordsbenzylammonium-(iH cis-l ,2-epoxypropyl )phosphonate, melting point 152to 155C.

EXAMPLE 5 Diethyl-(:)-( cis-l ,2-epoxypropyl)phosphonate A solution of1.94 g. (0.01 Mole) of diethyl-cis-propenylphosphonate in 550 ml. ofl,l,2-trichloro-l,2,2- trifluoroethane is degassed with a fine stream ofdry nitrogen from a gas spargcr and then irradiated under nitrogen with450 watt high pressure mercury immersion lamp surrounded by a 2 mm.thick cylindrical filter sleeve of corex glass which excludes light ofwavelength shorter than 260 mm. The reaction is followed by infraredexamination of aliquots until the carbonyl absorption of startingmaterial has disappeared (ca. 4 hours at room temperature). The solventis evaporated in vacuo and the residue distilled in vacuo giving purediethyl- (:)-(cis-l,2-epoxypropyl)phosphonate, boiling point 78 to82C./0.5 mm Hg.

EXAMPLE 6 Cis and trans-l-propenyl-(o-phenylene)phosphonate A pumpcirculated solution of 1,000 ml. of a 1 percent wt.- vol. solution ofo-phenylene cis-propenylphosphonate in absolute ethanol containing gramsof phenylcyclopropyl ketone as a photosensitizing agent, is irradiatedunder nitrogen through a 3 cm. thick pyrex filter with a 1,000 wattmercury-xenon arc lamp having a collimated beam of broad spectrum lightwhose focal point is the center of a quartz, water jacketed reactionvessel. Disappearance of starting material is monitored by disappearanceof the aldehyde proton nmr signal (ca. 8 hours at room temperature) inaliquots.

The ethanol and sensitizer are removed in vacuo leaving a gummy residuewhich crystallizes on trituration with benzene containing 1% petroleumether. Column chromatographyof crude material on silica gel usingmethanolbenzene eluent affords both cis andtrans-l-propenyl-(o-phenylene)phosphonate.

When in the above procedure methyl ethyl ketone, acetophenone, orbenzene is employed in place of phenylcyclopropyl ketone there isobtained the same 'cis and trans products in comparable yield.

The products obtained via the above processes may, if desired, beconverted to (cis-l,2-epoxypropyl)-phosphonic acid or its salts by anysuitable means as, for example, by hydrolytic means such as comprisestreating said esters with an aqueous solution of an acid such ashydrochloric acid or sulfuric acid at carefully controlled pH or with anaqueous solution of a base such as an alkali metal or alkaline earthmetal carbonate, bicarbonate, oxide or hydroxide or, alternatively, bytreatment with trimethylchlorosilane followed by aqueous hydrolysis; orby hydrogenolysis; or via the application of suitable reductive,displacement or oxidative means; or by treatment of the said esters oramides with a photochemical agent. The choice of a suitable method forthe conversion of the said esters and amides to(cis-1,2-epoxypropyl)phosphonic acid or its salts depends to a largeextent upon the character of the ester moiety comprising the phosphonateportion of the molecule. For example, when the ester is a dimethylester, the conversion to (cis-1,2-epoxypropyl)phosphonic acid is mostadvantageously conducted by treating the said ester withtrimethylchlorosilane followed by the aqueous hydrolysis of the silaneester intermediate thus obtained to afford the free acid. The alkysilaneester interchange is accomplished by refluxing the silane compound, suchas chlorotrimethylsilane, with the alkyl ester in an inorganic solventsuch as hexane, benzene and the like. In addition to the foregoing, thealkyl esters of (cis-l,2-epoxypropyl)- phosphonic acid and the arylanalogs thereof, including esters of mixed function such as(cis-1,2-epoxypropyl)-ph0sphonate wherein one ester moiety is derivedfrom phenol, naphthol and the like, may be converted to the free acid byalkaline hydrolysis. However, in view of the high degree of stability ofthe dialkyl esters it is not uncommon to find that the treatment of adialkyl (cis-l,2-epoxypropyl)-phosphonate with an aqueous solution of abase usually affords the monoalkyl ester intermediate and, therefore,the ultimate conversion of the alkyl diester to the salt or free acidnecessitates a second step, such as treatment with a photochemical agentor an acidic reagent in order to effect the removal of the remainingalkyl ester moiety.

Hydrogenolysis is particularly effective in converting the alkenylesters of (cis-l,2-epoxypropyl)-phosphonic acid to the correspondingfree acid and, preferably, the hydrogenation is conducted in thepresence of a Raney nickel catalyst and a base such as triethylamine,pyridine, dimethylaniline and the like, within a temperature range offrom about room temperature up to about 200C. Suitable inert organicsolvents which may be employed and the hydrogenation process include,for example, methanol, ethanol, ethyl acetate, acetic acid, dimethylether, diethyl ether, tetrahydrofuran, hexane, xylene, benzene and thelike.

The nuclear carbons comprising the epoxide ring in the instant products(11]) are asymetric in character and, therefore, the said products maybe obtained in the form of one or more of four optically active isomers.In this connection it should be noted that()-(cis-l,2-epoxypropyl)phosphonic acid and its salts are particularlyeffective in inhibiting the growth of pathogenic bacteria and,therefore, the preparation of that isomer constitutes a preferredembodiment of this invention.

The (--)-(cisl,2-epoxypropyl)phosphonic acid referred to herein rotatesplane-polarized light in a counter-clockwise direction (to the left asviewed by the observer) when the rotation of its disodium salt ismeasured in water (5 percent con-' centration) at 405 mu.

The designation cis used in describing the (1,2-epoxypropyl)phosphonicacid compounds means that each of the hydrogen atoms attached to carbonatoms 1 and 2 of the propylphosphonic acid are on the same side of theoxide ring.

The said (i)-. and (-)-(cis-l,2-epoxypropyl)-phosphonic acid derivativeand its salts are antimicrobial agents which are useful in inhibitingthe growth of gram-positive and gramnegative pathogenic bacteria. The()-form of (cis-1,2-epoxypropyl)phosphonic acid and particularly itssodium and calcium salts, are active against Bacillus, Escherichia,Staphylococci, Salmonella and Proteus pathogens, andantibiotic-resistant strains thereof. Illustrative of such pathogens areBacillus subtilis, Escherichia coli, Salmonella schottmuel- Ieri,Salmonella gallinarum, Salmonella pullorum, Proteus vulisms. Similarly,they can be used to separate certain microorganisms from mixtures ofmicroorganisms.

The compounds of this invention are also useful as a biostatic agent inpapermill white water. ()-(cis-l,2-epoxypropyl)phosphonic acid iseffective in reducing the bacterial population of white water when usedin concentrations as low as 0.8 percent. The activity is notedimmediately after addition of the compound, and is seen to persist for aperiod in excess of 48 hours.

What is claimed is:

l. A process for the preparation of a compound having the formula:

wherein X and Y can be the same or different and are selected from thegroup consisting of hydroxyl, lower-alkoxy, loweralkenyloxy, phenoxy,phenoxy substituted with loweralkyl,

loweralkanoyloxy, amine, amine substituted with loweralkyl, OM where Ois oxygen and M is the cation derived from a metal or an amine; whichcomprises treating a compound having the formula:

wherein X and Y are as defined above, with a source of ultravioletirradiation.

2. A process as defined in claim 1 in which the irradiation is effectedat a temperature of from 50 to C. in a solvent selected from the groupconsisting of loweralkanols; lowerketones, lower ethers, diethers andcyclic ethers; hydrocarbons of up to 8 carbon atoms; and aromatichydrocarbons of up to 8 carbon atoms.

3. A process as defined in claim 1 in which the mercury vapor lamp is ofan intensity greater than 200 watts.

4. A process as defined in claim 1 which includes photosensitizing agent5. A process as defined in claim 4 in which the photosensitizing agentis selected from the group consisting of loweralkonones, aromaticketones and aromatic hydrocarbons.

2. A process as defined in claim 1 in which the irradiation is effectedat a temperature of from 50* to 150*C. in a solvent selected from thegroup consisting of loweralkanols; lowerketones, lower ethers, diethersand cyclic ethers; hydrocarbons of up to 8 carbon atoms; and aromatichydrocarbons of up to 8 carbon atoms.
 3. A process as defined in claim 1in which the mercury vapor lamp is of an intensity greater than 200watts.
 4. A process as defined in claim 1 which includes aphotosensitizing agent.
 5. A process as defined in claim 4 in which thephotosensitizing agent is selected from the group consisting ofloweralkonones, aromatic ketones and aromatic hydrocarbons.